The conventional preparation method for producing plasma from blood in a clinical laboratory is centrifugation. Disadvantages of centrifugation include equipment cost and complicated handling, especially at large scale. In order for filtration to be a viable alternative to centrifugation, the method must yield plasma that is of the same quality as that of centrifuged plasma, which generally means that filtered plasma must have the same composition as centrifuged plasma. Such quality is not achieved without careful consideration of the different forces acting upon blood between centrifugation and filtration. Blood is a complex biological fluid whose composition is highly dependent on methods of manipulation. Known to the clinical laboratory are unwanted composition changes that may occur such as red blood cell lysis, clotting, and loss of antigen if the separation is incorrectly conducted. Specifically, the analyte of interest in the plasma may be at different concentrations between centrifugation and filtration due to the loss of analyte to the filter by adsorption. The amount of material lost to the surface of a filter depends on the surface properties and surface area of the filter. Filter membranes are typically made of materials such as polymers and glass fibers that because of their surface properties adsorb components of the plasma. Additional composition changes may occur, which have potential adverse effects. The amount of red blood cell lysis caused by the filtration must be negligible. The filtration process should not induce the fibrinogen formation, i.e., clotting. These are all common pitfalls of filtration methods, and ones that must be avoided in order to successfully replace centrifugation.
The amount of blood available for a clinical laboratory analysis is that volume from a blood collection tube, which is typically about 2 mL to about 10 mL. In order to be a viable alternative to centrifugation, a filtration method must yield sufficient plasma for the clinical laboratory analysis. The maximum amount of plasma available from blood is the difference in total volume and hematocrit. For example, with 4 mL of blood from a patient with 40% hematocrit, the total amount of plasma is 2.4 mL. Typical of all filtration methods, the entire plasma content of blood is not recoverable. The amount of plasma collected relative to the total available plasma is the plasma recovery efficiency. For example, if 1.2 mL of plasma from the available 2.4 mL is collected, then the plasma recovery efficiency is 50%.